Would it be possible for a planet-wide climate be set so that the temperature even at the poles would continually stay within a narrow range between 17 and 21 degrees Celsius?

(This is approximately 62 to 69 degrees Fahrenheit.)

In this scenario, the temperature would stay within this range even at the top of the tallest mountains on that planet.

Also, this would not be a scenario where, for example, the poles have the 17-or-so degrees and the equator has 21-or-so degrees. Rather, the 17-to-21 range would be present everywhere on the planet.

In this scenario the temperature could vary between any two spots (for what-ever reason). Thus we could, at any given time have e.g. 20 degrees Celsius at a location on the north pole, while at the same time having e.g. 18 degrees Celsius at some location on the equator.

This planet should preferably support human life, if that is possible.

Under what circumstances would this kind of evenly distributed temperature be possible?

$\begingroup$Does it have to be a planet that can support human life, or can it be any kind of planet?$\endgroup$
– GrandmasterBDec 8 '14 at 22:51

$\begingroup$@GrandmasterB: This planet should preferably support human life (although I am interested also about cases where it does not necessarily do so). Thanks, I have edited the question.$\endgroup$
– x457812Dec 8 '14 at 23:06

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$\begingroup$My solve-the-problem-without-answering-it-engine tells me to just make your planet arbitrarily small. If your planet is 1mm in diameter, it will be a pretty constant temperature by conduction. Needs to meet IAU definitions? Fine. Make it arbitrarily dense also.$\endgroup$
– imallettDec 9 '14 at 6:00

4 Answers
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There are two types of planets which are likely to exhibit small temperature ranges that I can think of: planets exhibiting a significant greenhouse effect and those with rapid rotation rates.

Greenhouse gases

Small temperature variations are found on planets with thick atmospheres. On these planets, the thickness of the atmosphere provides sufficient insulation to reduce or stop temperature drops at night. The same effect also allows for temperatures to come into equilibrium between the poles and the equator, so moving North or South also produces negligible changes in temperature.

A prime example of this is Venus, the surface of which is fairly isothermal, despite the fact that it revolves very slowly. Of course, for a habitable planet, it would need to be a bit further from the sun. With a thick atmosphere, even tidally locked planets can be fairly evenly heated. In this paper, the authors estimated less than a six degree temperature difference, regardless of position around the equator.

Venus is also estimated to have an isothermal layer that extends out to about 10km, so any mountains below that height will be about the same temperature as the ground. Higher mountains poke up into the next layer of the atmosphere, and have cooler tops. (And Venus has these! It's tallest is around 11km high, which is bigger than Everest. Astronomy is awesome!)

Rotation rate

A rapidly spinning planet would also be more evenly heated, since there would be less time for the surface to heat up during the day or cool down at night. It would, however, have some powerful weather systems. Spinning rapidly would also only address the day/night fluctuations, not fluctuations based on altitude.

$\begingroup$Altitude variations are easy: there isn't necessarily a requirement for the planet to have altitude variations. Without any plate tectonics, there shouldn't be any large mountains or valleys. You'd still have trouble with different latitudes though.$\endgroup$
– BobDec 9 '14 at 10:48

There are three huge reasons for temperature fluctuations on a planet: The parent star, axial tilt and the curvature of the planet.

The star. There's normally a huge difference on temperatures between day and night on a planet. On the side facing the star, light radiated from the star heat up the planet considerably. The night side doesn't have as much light beamed at it, and so it's a lot cooler. In order to maintain a temperature equilibrium, you have to get rid of day and night. The obvious way to do this is to place the planet where there isn't any star. It's a rogue planet, floating freely through space. The problem is, these planets aren't too conducive to life.

The alternative is to eliminate the night completely by making sure that both sides are continuously illuminated. You'd want a binary star system to do this (unless you plan on making some giant artificial light-emitting structure), and you'd want to set it up every specifically. Take two stars that are roughly the same - in mass, luminosity, age, and every other major characteristic. Also, make them G2 stars, like the Sun. Now place the planet in the center of mass of the system. In theory, the planet will be in an equilibrium, and will receive light from both sides as the stars "orbit" it. This arrangement, though, is unstable.

Yet another way to do this would be to have one star orbiting another. The main star is massive, while the small star is less massive. Place the planet in L1, the Lagrangian point of the smaller star, which is between the two. The planet should stay right in the middle of the two stars. Make the placements so the planet is far enough away from the more massive (and likely more luminous) star, and closer to the other star, so it maintains a balance of temperature.

As pointed out below, L1 is inherently unstable, as are many of the Lagrange points. There is a way around this, which is to put the planet in a halo orbitaround L1 (though technically the planet wouldn't be orbiting L1. This orbit would also be unstable, so you'd need to make adjustments using a technique known as station keeping. This probably wouldn't work for a planet - after all, you'd have to attach huge rockets to it! Halo orbits can't exist in the Solar System or similar $n$-body systems, so Lissajous orbits may be used instead. These are also a little unstable, and nearly impossible for a planet, even with artificial assistance.

Here's what this kind of orbit would look like:

Image in the public domain.

Axial tilt. The tilt of the Earth's axis is the reason we have seasons. Even though we're closer to the Sun at some times in the year, this makes no difference whatsoever in our temperature. Axial tilt does, and it means that, no matter which sides of the planet are lit up, part of it will be receiving more light than the other.

The obvious solution is to get rid of axial tilt. It is rare (in our Solar System) for this to be the case with a body. Most objects have a lot of tilt; the Earth's is around 24 degrees. Your planet needs to have none whatsoever.

Curvature. This goes along partly with point number 2. Earth is close to a sphere (technically, an oblate spheroid), which means that light hits some parts less directly than others. This is a problem that can't be averted by the binary star solution, of by the zero-tilt solution. In fact, there doesn't appear to be any natural solution.

You need to surround the entire planet with something emitting light uniformly. You could create a huge sphere (similar to a Dyson sphere) around the planet and light up the inside. That's really good for control. I can't think of any feasible way this could happen naturally, though. You need artificial intervention.

$\begingroup$"In order to maintain a temperature equilibrium, you have to get rid of day and night." - Would it not be possible to obtain day-night equilibrium through e.g. some greenhouse gases?$\endgroup$
– x457812Dec 8 '14 at 22:59

$\begingroup$@x457812 Creative, but the gases would likely dissipate and spread throughout the atmosphere, thereby affecting the temperature globally. You could try to block out the Sun, but again, that has a negative effect on life on the planet.$\endgroup$
– HDE 226868♦Dec 8 '14 at 23:00

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$\begingroup$@MikeNichols I suppose you're right. The issue is that there's no mechanism that could explain that sort of chaotic rotation. But if there was, that would totally work.$\endgroup$
– HDE 226868♦Dec 9 '14 at 3:22

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$\begingroup$Minor point: the L1 Lagrange point is also unstable.$\endgroup$
– trichoplaxDec 9 '14 at 9:40

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$\begingroup$@githubphagocyte Yeah, I want to up-vote this answer but can't because of that :( L2 or L3 point might work though, you wouldn't get complete day coverage but nights would be pretty short.$\endgroup$
– Tim BDec 9 '14 at 10:03

I recall a Hal Clement story about a planet where the atmosphere was at the triple-point, with air and water being ambiguous or suddenly changing.

So, how about some extreme or exotic process that evens out the temperature rapidly? Normal wind and current moves heat around; some super phase-change substance could do so fast enough to make differences in insolation irrelevent. At the very least, it could keep the temp. constant with altitude in one location.

The problem with this requirement for a 1% temperature uniformity on a planetary scale is that heat conducts slowly relative to the size of a planet. Conduction, winds, you name whatever temperature balancing mechanic you wish to name and it will take time to move the temperature away from high noon at the equator to midnight at one of the poles.

All of the energy sources to a planet are point sources (vulcanism) or solar (incident from one direction). This gives an innate directional component to your heating that playing games with two stars just isn't going to fix.

There have been times when the climate on earth was more uniform than the present - in the age of the Dinosaurs alligators and turtles were found even in high latitudes. But three degrees at all times just isn't possible.